[TheWrongGrape]

Just did this for my site running 4.3.1

In send_to_friend.php, add this code around line 78 below $mail_smarty->assign ("product", $product_info);

$mail_smarty->assign ("msg", $msg);

In skin1/customer/main/send_to_friend.tpl add a new row before the row containing the submit button:

    <tr>
      <td class="data-name"><label for="msg">Optional Message</label>:</td>
      <td></td>
      <td>
        <textarea id="msg" type="textarea" name="msg" /></textarea>
      </td>
    </tr> 

Style and size the textarea however you like.

Lastly, in either your HTML or text email tpl file (I use skin1/mail/html/send2friend.tpl), add:

{if $msg}
$msg
<br/>
{/if}

Hope this helps. Kudos to Shan's old post for giving me the help I needed.